The present invention relates to fluid controllers, and more particularly, to fluid controllers which are of the static load signal type.
For many years, hydraulic circuits have been provided with xe2x80x9cload sensingxe2x80x9d capability, in which the primary control device (typically, a valve) of the circuit is provided with a primary flow control orifice, and the device communicates a load signal from a predetermined location in the device, generally downstream of the primary flow control orifice. The load signal is then utilized to control a device, such as a variable displacement pump, such that the fluid output of the pump varies in accordance with changes in the load signal. Load sensing technology has been especially useful, and commercially desirable, in hydraulic circuits used on vehicles in which the primary device generating the load signal is a hydrostatic power steering device, also referred to as a fluid controller or a steering control unit (SCU).
From the inception of load sensing technology, the load signals were typically xe2x80x9cstaticxe2x80x9d, i.e., there was no actual fluid flow in the load signal circuit, but instead, the load signal being sensed was merely a static pressure. Subsequently, in order to improve response time of hydraulic circuits, and the various components of the circuit, those skilled in the art developed xe2x80x9cdynamicxe2x80x9d load signal technology, in which pressurized fluid is continually communicated into the load sensing circuit, and typically the dynamic signal fluid recombines with the main flow path, downstream of the main variable flow control orifice. An example of a dynamic load signal system is illustrated and described in U.S. Pat. No. 4,620,416, assigned to the assignee of the present invention and incorporated herein by reference.
Although dynamic load signal technology and devices have enjoyed widespread commercial success, and have been very satisfactory in performance, there are frequently applications for load sensing devices wherein providing the device with dynamic signal capability is not a viable option. For example, in the case of a load sensing SCU, if the SCU is to be part of an overall vehicle hydraulic system which has a load sensing pump as its source, and which includes a number of static load signal devices, then the SCU must also be a static load signal device. However, in many such situations, the vehicle OEM may have used dynamic load signal devices in some of its vehicles, and may have come to expect a certain level of performance (and especially, response time) from the hydraulic circuits utilized on its vehicles. Then, when the OEM is required (for the reasons noted above) to utilize a device which is of the static signal type, the performance is likely to be less than what the OEM has come to expect.
However, the assignee of the present invention has determined, in connection with the development of the present invention, that in certain hydraulic circuit applications, a static load signal SCU may be displaced from its neutral position all the way to a maximum displacement position before the load signal pressure in the circuit (and controlling the pump) builds up to a pressure level sufficient to cause the pump to provide the amount of fluid flow required to operate the steering circuit. When the above-described scenario occurs, much of the steering effort and movement of the steering wheel by the operator does not result in any substantial amount of turning of the steered wheels, thus appearing to the operator as at least a momentary loss of steering capability. The operator would rotate the steering wheel and the SCU valving would move to full valve displacement (deflection) until the mechanical stop would be engaged, after which the operator would, for a brief period, have only manual steering.
One potential, and seemingly obvious, solution to the above-described problem of slow response time in a static load sensing circuit is simply to increase the xe2x80x9cstandbyxe2x80x9d pressure generated by the pump. This may be done in any of several ways, one of which is to simply increase the spring force in the flow and pressure compensator of the pump which determines the flow and pressure output from the pump when the circuit being fed by the pump is not demanding any flow (i.e., is merely in a xe2x80x9cstandbyxe2x80x9d condition). However, in connection with the development of the present invention, it was determined that simply increasing the standby pressure of the pump did not appreciably improve the response time of the SCU.
Accordingly, it is an object of the present invention to provide an improved fluid controller of the static signal type having substantially improved response time, in comparison to prior art static signal controllers.
The above and other objects of the invention are accomplished by the provision of a fluid controller operable to control the flow of fluid from a source of pressurized fluid to a fluid pressure operated device, the source of fluid including pressure responsive means for varying delivery of fluid to the controller in response to changes in a static load signal. The controller includes housing means defining an inlet port for connection to the source of fluid, a control fluid port for connection to the fluid pressure operated device, a fluid return port for connection to a reservoir, and a load signal port for connection to the pressure responsive means. Valve means is disposed in the housing means and defines a neutral position and an operating position providing fluid communication from the inlet port to the control fluid port. The pressure responsive means for varying delivery of fluid is selected to provide a predetermined standby pressure of the source of pressurized fluid. The valve means defines a variable load sense drain orifice communicating from the load signal port to the fluid return port, the load sense drain orifice having a maximum flow area when the valve means is in the neutral position, and a decreasing flow area as the valve means is displaced from the neutral position toward the operating position.
The improved fluid controller is characterized by the valve means defining a variable load sense boost orifice communicating from the inlet port to the load signal port. The load sense drain orifice is sized, relative to the load sense boost orifice, to generate the static load signal to have a predetermined relationship to the predetermined standby pressure of the source of pressurized fluid.